The true clinical significance of impaired oxygen diffusion is debatable, except during exercise and other high-output states. The diffusing capacity is defined as
DLq=Qi uptake I'/'AOj mean pulnwnOTapilhry/Yij). (4)
The capillary transit time, which equals the pulmonary capillary blood volume divided by the pulmonary blood flow (or cardiac output), determines the mean pulmonary capillary PO2. Therefore the factors that contribute to a decreased oxygen diffusion capacity are shorter capillary transit time, decreased area or increased thickness of the alveolar capillary membrane, or a reduction in the pulmonary capillary blood volume.
Increased cardiac output results in decreased contact time between the red cell and the alveolar interface or decreased capillary transit time. The normal mean transit time is approximately 0.75 s. However, it may decrease to 0.1 s during heavy exercise in normal subjects, and there is a wide distribution of values around the mean.
Other hyperkinetic states, such as sepsis or thyrotoxicosis, also have short transit times. The rapid transit time and VtQ mismatch characteristically found in septic patients contribute to the hypoxemia found in this population. Figure^, shows the effect of shortened transit time on equilibration of oxygen tension between the alveolus and the red cell.
Fig. 2 Increased cardiac output (reduced contact time) and thickened alveolar membrane both affect diffusion of oxygen from the alveolus into the blood. (Reproduced with permission from We§t..i1998).)
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